This invention is drawn to the field of high frequency tunable oscillators, and more particularly, to a coherent array of Josephson oscillators.
The ac Josephson effect refers to the fact that when two superconductors are separated by a barrier, such as an insulating layer in a tunnel junction or a microbridge in a weak link, and a voltage (V) is established across the two superconductors, for example, by passing a biasing current therethrough, a periodic oscillating current of frequency f, which is associated with Cooper pairs passing through the junction, can be detected by the electromagnetic radiation of frequency f that it generates. The frequency (f) of this Josephson current is given by the relation f=2 eV/h; where e is the electron charge and h is Planck's constant.
The ac Josephson effect is also exhibited by exposing the junction to external radiation at another frequency, f'. In this case, it is found that a graph of dc current versus voltage for the junction has current steps at values of the voltage for Josephson frequencies that are integral multiples (n) of the external frequency (f') according to the relation V=nhf'/2e; whereby the junction can be said to lock to the external radiation by a process similar to injection locking.
By combining Josephson oscillators into an array in such a manner that the microwave radiation from each adds coherently, the low power levels and broad linewidths that characterize single Josephson oscillators are overcome. Possible applications for such coherent multijunction arrays include direct use as microwave and millimeter wave generators and detectors, parametric amplifiers, voltage standards and other uses.
One technique that can be used to provide coherent coupling in a multijunction array, reported in an article entitled: "Generation of Coherent Tunable Josephson Radiation at Microwave Frequencies with Narrowed Linewidths", by Varmazis et al, some of the authors of which are the present applicants, appearing at pp. 357-359, Appl. Phys. Lett. 33(4) (1978), incorporated herein by reference, comprises a resistive shunt around a series connected array of two (2) junctions. Although this technique demonstrates that the junctions lock to a common voltage and phase, the variation in characteristics between actual junctions makes it difficult to operate larger arrays at the same frequency without individual current biases.
An approach that can be used to overcome the problem of non-identical junctions, reported in an article entitled: "Flux Modulated Coherent Radiation from Arrays of Josephson Microbridges Coupled by Superconducting Loops" by Sandell et al, some of the authors of which are the present applicants, appearing at pp. 462-464, IEEE Trans. Magnetics, Vol. Mag. 15, No. 1, (1979), incorporated herein by reference, comprises a multijunction array of Josephson oscillators in an arrangement of interlocking dc SQUID's (Superconductive Quantum Interference Device), which assures that the average frequency of each of the array junctions is always equal even for non-identical junctions. Phase coherence between adjacent junctions is obtained by controllably varying an externally applied magnetic flux. Because of the condition of flux quantization in the individual SQUID loops, however, the induced Josephson oscillations add in random phase. That is, it has been found that the dc SQUID configuration controlled by the externally applied flux provides multijunction arrays with frequency synchronization but not with phase coherence.